Flu Season May Be Starting Early as H1N1 and New Mutation Raise Concerns

Flu season could be starting earlier than usual in the Northern Hemisphere, according to early surveillance data. The trend is prompting some people to reconsider when to get their annual influenza vaccination.

In places including the United Kingdom and the US West Coast, positive influenza test rates are unusually high for this time of year. In Japan, influenza was prevalent from August to early September and forced some schools to close as summer vacation ended.

Scott Hensley, a virologist at the University of Pennsylvania in Philadelphia, said that if cases on the US West Coast continue rising at their current rate, “this will be incredibly early” for the United States.

So far, H1N1 is causing most of the cases in the United States, a change from last year, when H3N2 dominated. Sequence data from these early cases also point to new H1N1 mutations that are raising concerns among experts.

Early flu activity does not necessarily mean a severe season

The findings are based on a relatively small number of positive test results compared with the amount of testing available during peak flu season. Researchers say it is therefore too early to predict the overall trajectory of the epidemic.

“An earlier start doesn’t necessarily mean a tougher season,” said James Hay, an epidemiologist at the University of Oxford in the United Kingdom. Still, “something is happening,” epidemiologist Caitlin Rivers wrote in her September 27 newsletter, “Infectiousness.”

The early activity has also led Rivers, a researcher at Johns Hopkins University in Baltimore, Maryland, to reconsider her personal protection strategies. Influenza season in the United States typically peaks in December and January, so she had planned to wait until mid-to-late October to receive a flu vaccine for the best protection. She is now considering vaccination as soon as possible.

New H1N1 mutation G155E draws researchers’ attention

Seasonal influenza is usually caused by a combination of two influenza A subtypes—H3N2 and H1N1—and influenza B viruses. Because influenza viruses mutate over time and population immunity changes after vaccination and infection, the dominant virus can vary from one flu season to the next.

US data showed that, in the week ending September 26, 96% of positive cases that had been subtyped were H1N1. H3N2 accounted for almost 4%, while influenza B was not detected.

This differs from last year, when a new H3N2 variant known as subclade K emerged. The virus became dominant worldwide after accumulating mutations that made people more susceptible to infection.

Are mRNA influenza vaccines more effective than regular seasonal vaccines? What the science says

The H1N1 mutation attracting attention this year is G155E. Alex Greninger, a virologist at the University of Washington in Seattle, said his laboratory detected G155E in 13 of 17 H1N1 samples whose genetic sequences were complete enough to identify the mutation. The samples were collected from Seattle hospitals and clinics over the past few months.

The mutation was previously identified in an analysis led by Jesse Bloom, a virologist at Fred Hutchinson Cancer Center in Seattle. The study was posted on the bioRxiv preprint server last month, ahead of peer review.¹

Bloom and his colleagues tested how well antibodies in blood samples from 260 people in Australia and the United States could recognize and block infection from 148 influenza A viruses expected to circulate by mid-2026.

“We found that a large number of strains with the G155E mutation were, in some cases, poorly recognized by existing human antibodies,” Bloom said. Antibodies in samples from teenagers and young adults were particularly poor at recognizing the variant. This could be because many people in these groups have not previously encountered H1N1 viruses with similar characteristics.